1. Field of the Invention
The invention relates to a method of manufacturing an optical fiber comprising a core portion with a dopant and a surrounding optical cladding portion, comprising the steps of drawing a fiber from a molten extremity of a silica preform and moving the fiber along means for coating it with a protective sheath. In particular, the invention relates to an optical fiber having reduced polarisation mode dispersion.
2. Discussion of the Related Art
The term "polarisation mode dispersion" (PMD) refers to the dispersion of signals carried by an optical fiber (particularly a single mode fiber), as a result of birefringence in the fiber's core portion. This birefringence is caused by imperfections in the fiber, such as slight non-circularity of its core cross section, asymmetrical lateral stress, etc., and manifests itself in dissimilar refractive indices for a carried signal's two orthogonal polarisation modes. In the case of a perfect fiber devoid of PMD, these two modes propagate independently of one another at a common velocity. However, in the presence of PMD, the relative phase of the two modes changes continually, returning to a particular configuration at certain spatial intervals along the fiber. The average such interval is referred to as the beat length L.sub.p of the fiber, and has a typical magnitude of the order of 1 m (meter).
A method as stated in the opening paragraph is known from U.S. Pat. No. 5,298,047, wherein, subsequent to the fiber coating step, the moving fiber is caused to pass over a series of pulleys. By continually canting the rotational axis of at least one of these pulleys back and forth in an irregular manner (i.e. with a variable frequency), the fiber is subjected to an oscillatory torque which causes the entire length of fiber between the canting pulley and the preform to twist back and forth. Such twisting of warm, uncoated fiber impresses an oscillatory axial spin upon its constituent material, and this spin subsequently becomes "frozen" into the fiber as it cools. The deliberate presence of such variable spin-related stress in the fiber produces continual mode-coupling between the orthogonal polarisation modes of a carried signal, thereby inhibiting the accumulation of a significant phase lag between the two modes, and consequently causing a significant reduction in the fiber's PMD.